1. Field of the Invention
The present invention relates to a waste heat utilization device and a control method thereof. For example, the waste heat utilization device recovers power by using waste heat of a heat generating unit, and can be suitably used for a vehicle having an internal combustion engine.
2. Description of the Related Art
A waste heat utilization device is described in JP-A-2005-307951 (corresponding to US 2005/0235670 A1), for example. The waste heat utilization device includes a refrigeration cycle for air-conditioning, and a Rankine cycle that recovers waste heat from a heat generating unit such as an internal combustion engine. The refrigeration cycle includes a compressor that is driven by power of the engine to compress and discharge refrigerant, and the Rankine cycle includes an expansion unit that is operated by expansion of the refrigerant heated by the waste heat. The waste heat is generated from the heat generating unit and is obtained during a cooling operation of the heat generating unit. Furthermore, the Rankine cycle is constructed to use a condenser (radiator) in common with the refrigeration cycle.
Furthermore, a stand-along operation (single operation) of the refrigeration cycle, a stand-along operation (single operation) of the Rankine cycle and a simultaneous operation (combination operation) of the refrigeration cycle and the Rankine cycle are selectively performed in accordance with an air-conditioning necessity, a heat recovering possibility of the waste heat, etc.
However, in a case where cooling load is large in the refrigeration cycle in the summer season, when the refrigeration cycle and the Rankine cycle are simultaneously operated in the waste heat utilization device, energy loss may be caused in the whole cycle of the waste heat utilization device even when waste heat is recovered in the Rankine cycle. The problems will be described in detail as follows based on the experiments performed by the inventors of the present application.
FIG. 12 is a graph showing a relationship between the cooling load of the refrigeration cycle and a driving power of the compressor when only the refrigeration cycle is operated (single operation), and FIG. 13 is a Mollier diagram (p-h diagram) of the refrigeration cycle used in FIG. 12. In FIG. 13, DP is the driving power of the compressor. Next, FIG. 14 is a graph showing a relationship between the cooling load of the refrigeration cycle and the driving power of the compressor when both the refrigeration cycle and the Rankine cycle are simultaneously operated (combination operation), in addition to the single operation of FIG. 12.
As shown in FIG. 14, the driving power of the compressor becomes very high in the combination operation as compared with the single operation of the refrigeration cycle, because heat from the refrigeration cycle and heat from the Rankine cycle are introduced into the common condenser. Thus, in the combination operation of both the refrigeration cycle and the Rankine cycle, the condensation pressure of the condenser becomes higher as compared with the single operation of the refrigeration cycle, as shown in FIG. 15. In FIG. 15, CP indicates the increase of the condensation pressure in the combination operation as compared with the single operation, EO indicates the output power of the expansion unit in the combination operation, and DP1 indicates the power deterioration in the compressor as compared with the single operation.
A compression ratio (i.e., a ratio of the discharge pressure to the suction pressure) of the compressor becomes higher as the condensation pressure CP increases, thereby the driving power of the compressor is increased. FIG. 16 shows the driving energy E1 for driving the compressor and the recovered energy E2 recovered in the Rankine cycle, in accordance with a change of the cooling load, during the combination operation. In FIG. 16, Z indicates the balance point between the driving energy E1 and the recovered energy E2, and the cooling load generally becomes larger as an outside air temperature increases. As shown in FIG. 16, in the combination operation of the waste heat utilization device, even when the energy is recovered in the Rankine cycle, the energy balance between the recovered energy E2 and the driving energy E1 becomes minus when the cooling load is larger than a predetermined value, thereby deteriorating the fuel consumption efficiency in the engine.